Electrochromism generally refers to the reversible change in the optical properties of a material upon application of a potential. In particular, electrochromic materials exhibit a reversible color change due to an electrochemical reduction-oxidation (redox) reaction caused by application of an electric field.
Electrochromic materials may include both organic and inorganic materials. For instance, electrochromic materials may include inorganic metal oxides, conductive polymers, and organic materials such as viologen, anthraquinone, phenocyazine, etc. Conjugated, conductive polymers may be particularly useful as electrochromic materials due to their wide range of color tenability, ease of processing, low operational voltages, and color retention.
Electrochromic materials are commonly used in electrochromic devices. A typical electrochromic device includes a four layer assembly: (i) a first transparent electrically conductive film, (ii) an electrochromic material deposited on the first conductive film, (iii) a second transparent electrically conductive film spaced apart from the first conductive film, and (iv) an ionic conductive medium (electrolyte) disposed between the first and second conductive films. It has been found that the inclusion of only one electrochromic film may result in electrochromic devices with shortened lives. Accordingly, a second electrochromic material film may be deposited on the second conductive film noted above to facilitate completion of the redox process in the device and reduce or eliminate degradative reactions in the electrolyte. This five layer assembly may be used to obtain two mixed colored states or, may, using two materials with complementary optical characteristics, enhance the contrast between the previously defined states.
There are several problems associated with known electrochromic devices and/or the components thereof. U.S. Pat. No. 5,441,827 (Graetzel, et al.) discloses one such electrochromic device in which a layer of an electrochemically active semiconductor oxide is coated onto the surface of an electrode, where the semiconductor oxide layer is in the form of a porous nanostructure formed of sintered colloidal particles. The device in the '827 patent also uses a lithium salt in an organic solvent as a liquid electrolyte. This device has several disadvantages, such as a low quenching rate, the present of residual images after quenching, decomposition of organic materials during repeated developing/quenching cycles, evaporation and exhaustion of the organic solvent, leakage of the liquid electrolyte, and inability to form thin film or film-shaped products.
U.S. Pat. No. 5,827,602 (V. R. Koch, et al.) discloses an aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl-EMICI) ionic liquid including a strong Lewis acid as a liquid electrolyte. As the AlCl-EMICI ionic liquid has no vapor pressure, it reduces and/or eliminates issues related to electrolyte exhaustion and decomposition. However, disadvantages associated with AlCl-EMICI include the emission of toxic gases upon exposure to a small amount of moisture or oxygen, a high reactivity with organic/inorganic compounds added to the electrolyte, and decomposition at temperatures of 150° C. or higher.
U.S. Pat. No. 6,667,825 (Wen Lu, et al.) discloses an electrochromic device utilizing two conjugated polymer coated ITO-coated glass electrodes, and an ionic liquid such as [BMIM][BF4] as the electrolyte. The [BMIM][BF4] liquid electrolyte of the '825 patent does not include a Lewis acid, which results in improved stability and lifespan of the electrochromic device. Further, the electrochromic device of the '825 patent may avoid, at least in part, issues arising with residual images after quenching and electrolyte decomposition that are typically are found in devices using organic solvent-based liquid electrolytes and ionic liquid electrolytes containing a Lewis acid. However, the electrochromic device of the '825 patent is still subject to problems associated with leakage of the liquid electrolyte, and the inability to be formed into thin films and film-shaped products.
Accordingly, there is a need in the art for an improved electrolyte suitable for use in electrochromic devices.